1. Field of the Invention
The invention relates to a line fault monitoring apparatus which detects a short circuit of lines and isolates the short-circuit portion from a power source side.
2. Description of the Related Art
Conventionally, such a line fault monitor which is called a short-circuit isolator is disclosed in Unexamined Japanese Patent Publication No. SHO. 63-19098.
In such a line fault monitor, lines in a terminal side are connected to a receiver in a loop manner, and one or more short-circuit isolators are provided in the loop lines. If a short circuit occurs in the lines, the short-circuit isolators positioned in both sides of the short-circuit portion disconnect the short-circuit side. Then, the power is supplied to the remaining normal lines so that the monitoring condition is maintained.
As shown in FIG. 15 which is a system explanation diagram of the Unexamined Japanese Patent Publication No. SHO. 63-19098, the line fault monitor includes following elements. A receiver 301 stores a power source and receiving circuit 301a therein. A pair of power/signal lines 302a and 302b are extended from the receiver 301 from terminals 311, 312. The lines are returned to the receiver 301 in a loop form. Between the lines 302a and 302b connected to the receiver 301 in a loop form, terminal loads 303 which are relays, sensor or the like is connected in parallel, and further, line fault monitors 304a and 304b are inserted and connected in each group in which a predetermined number of the terminal loads 303 are provided. According to the insertion and connection of the line fault monitors, the loop connected lines 302a and 302b are separated into sections of L1, L2, and L3. Further, the receiver 301 is provided with a pair of line fault monitors 315a and 315b, each having power source side terminals 309 and 310, and which are connected each other and connect to a receiver circuit 301a, and having respective load side terminals 311 and 312 of the line fault monitor 315a and 315b as loop connection terminals to the receiver 301.
Further, Unexamined Japanese Patent Publication No. SHO. 62-73400 discloses a alarm apparatus in which a pair of lines which are power line and power/signal line are separated in a plurality of sections and an isolating circuit is provided in each section. If a short circuit occurs in one section, the section is isolated by the isolating circuit so as to prevent shutting down the system. In addition, the isolating circuit includes an latching relay having an isolation contact inserted in the line. Furthermore, the Publication suggests an indicator which indicates the moving state of the latching relay.
As shown in FIG. 16 which is a block diagram of the Unexamined Japanese Patent Publication No. SHO. 62-73400, the alarm apparatus includes a central monitor RC, a pair of lines that are power line L and power/signal line C both extending from the central monitor RC, a plurality of terminals SN connected between the lines L and C, a plurality of isolating circuits inserted between the lines L and C, and a plurality of isolation circuit DX which are inserted between the pair of lines L and C so as to divide the lines in a plurality of zones. The pair of lines L and C may be loop form lines as indicated by broken lines.
Thus constructed alarm apparatus detects a short circuit of the lines following manner. The terminals SN are supplied a power from the central monitor RC through a pair of lines L and C so that the terminal SN is in monitoring state. The terminal SN sends a detection signal detected by a sensor of fire, gas or the like or a judgement signal based on thus detected signal to the central monitor RC. The central monitor RC conducts a fault judgement based on the signal sent from the terminal SN and, if necessary, conducts a alarm indication.
If a short circuit occurs somewhere on the lines L and C, the power can not be supplied to all of the terminals SN, thereby downing the whole system. However, an isolation circuit which is the nearest one to the short-circuited zone isolates the short-circuited zone. Accordingly, remaining system including terminals SN maintain a normal monitoring state.
However, according to the conventional line fault monitor disclosed in Unexamined Japanese Patent Publication No. SHO. 63-19098, when a short-circuit occurs, the downing of the whole system can be prevented but the position at which the short circuit occurs cannot be specified. Therefore, there exists a problem in that the restoration requires much labor and a long time.
Further, the conventional alarm apparatus monitor disclosed in Unexamined Japanese Patent Publication No. SHO. 62-73400 can detect the operation of the isolation circuit, but can not determine a line which side of the isolation circuit is short-circuited.
Accordingly, in case of restoring the short-circuited line, the voltages on the lines in both sides of the isolation circuit have to be measured so as to specify the short-circuited line. Therefore, the conventional alarm apparatus has the following problems. If a measurement instrument is connected to a line in a normal state to measure the line voltage or the operation error of the instrument occurs in the measurement, noises may occur in the line which may cause of a transmission trouble or a false signal. In addition, when the power of the alarm system is off to inspect the lines, the detection of a fire or the like can not be detected during the inspection.